In many organic reactions, the O-2 activation process involves a key step where inert ground triplet O-2 is excited to produce highly reactive singlet O-2. It remains elusive what factor induces the change in the electron spin state of O-2 molecules, although it has been discovered that the presence of noble metal nanoparticles can promote the generation of singlet O-2. In this work, we first demonstrate that surface facet is a key parameter to modulate the O-2 activation process on metal nanocrystals, by employing single-facet Pd nanocrystals as a model system. The experimental measurements clearly show that singlet O-2 is preferentially formed on {100} facets. The simulations further elucidate that the chemisorption of O-2 to the {100} facets can induce a spin-flip process in the O-2 molecules, which is achieved via electron transfer from Pd surface to O-2. With the capability of tuning O-2 activation, we have been able to further implement the {100}-faceted nanocubes in glucose oxidation. It is anticipated that this study will open a door to designing noble metal nanocatalysts for O-2 activation and organic oxidation. Another perspective of this work would be the controllability in tailoring the cancer treatment materials for high O-1(2) production efficiency, based on the facet control of metal nanocrystals. In the cases of both organic oxidation and cancer treatment, it has been exclusively proven that the efficiency of producing singlet O-2 holds the key to the performance of Pd nanocrystals in the applications.